1. Field of the Invention
The present invention relates to a method for driving and controlling a thermal printer effecting printing on a print sheet by means of a thermal head.
2. Description of the Prior Art
FIG. 5 shows an example of the thermal printer generally used. Almost at the central portion of a frame 1 of the printer there is disposed a plate shaped platen 2 so that its printing surface is almost vertical. Further before and under the platen 2 in the frame 1 stated above there is disposed a carriage shaft 3 so as to be parallel to the platen 2. Still further a flange shaped guiding portion 4 is formed at the front edge of the frame 1 and a rack 5 is formed at the inner edge of this guiding portion 4. A carriage 6 is mounted on the carriage shaft 3 and the guiding portion 4, movably forward and backward along the carriage shaft 3 and the guiding portion 4, driven by a motor engaged with the rack 5. A thermal head 7 is mounted at the extremity portion of the carriage 6 so as to be opposite to the platen 2 and a ribbon cassette (not shown in the figure) is mounted on the upper surface of the carriage, in which an ink ribbon is located, which ink ribbon is guided between the thermal head 7 and the platen 2 by the ribbon cassette.
Further, behind the platen 2, is formed a sheet insertion opening 8 for forwarding a sheet (not shown in the figure) before the platen 2. At a part of the sheet insertion opening 8 there is disposed a sheet forwarding roler 9, which is driven by a motor not shown in the figure to forward the sheet with a predetermined speed. Under this sheet forwarding roller 9 there is disposed rotatably a thrusting roller 10 thrust with pressure on this sheet forwarding roller 9 so that the sheet inserted through the sheet insertion opening 8 is held between the sheet forwarding roller 9 and this thrusting roller 10 to be forwarded.
FIG. 6 shows a prior art driving mechanism for the carriage and a prior art driving mechanism for the thermal head in such a printer. Within the carriage 6 there is disposed a carriage driving motor 11 and an output pinion 12 of this carriage driving motor 11 is engaged with the rack 5 through a gear 13 so that the carriage 6 is movable forward and backward along the carriage shaft 3 by the normal and reverse rotation of the carriage driving motor 11. On the platen 2 side of the carriage 6 a fixing member 14 for the thermal head 7 is mounted pivotably through a fulcrum 15 and the thermal head 7 is mounted at the extremity portion of this fixing member 14. One end portion 17a of a cam follower 17 formed in a bell crank shape and mounted rotatably around a fulcrum 16, acting as a driven joint is brought into contact with the fixing member 14 of this thermal head 7 and the other extremity portion 17b is engaged slidably with an endless cam groove 19 formed in a cam member 18 acting as a driving joint. An output pinion 23 of a motor 22 for moving upward and downward the thermal head mounted on the carriage 6 is engaged with a gear 20 formed on the outer periphery of this cam member 18 in one body therewith through a transmission gear 21 so that the cam member 18 is rotated by the rotation of the motor 22; the cam follower 17 is rotated, accompanied by this cam member 18; the fixing member 14 is thrust with pressure on the one extremity portion 17a of the cam follower 17 by the rotation of this cam follower 17 to be moved forward and backward with respect to the platen 2; and thus the operation of thrusting the ink ribbon and the recording sheet on the platen 2 by the thermal head 7 and removing them is carried out.
In addition, a motor for driving an ink ribbon rewinding bobbin 24 for rewinding the ink ribbon (not shown in the figure) is mounted also on the carriage 6, separately from the motor 22 described previously for moving the thermal head upward and downward and a rewinding gear 27 disposed coaxially to the ink ribbon rewinding bobbin 24 is engaged with an output pinion 25a thereof through a transmission gear 26. Further a sensor (not shown in the figure) for sensing the up- and downward movement of the thermal head is disposed and the movement of the ink ribbon rewinding motor 25 is controlled by this sensor, corresponding to the up- and downward movement of the thermal head 7.
In the prior art printer described above, the sheet is inserted through the sheet insertion opening and this sheet is put between the sheet forwarding roller 9 and the thrusting roller 10. In this way, the sheet is forwarded in the direction perpendicular to the movement direction of the carriage 6 by driving the sheet forwarding roller 9 so as to rotate it. On the other hand, the motor 22 for moving the thermal head upward and downward to thrust the thermal head 7 on the sheet with a predetermined thrusting force. In this state the motor 11 for driving the carriage is driven to move the carriage 6 and at the same time the ribbon rewinding motor 25 is driven to rotate the rewinding bobbin 24 and to rewind the ink ribbon in the ribbon cassette. In this way, desired printing is effected on the sheet by driving the thermal head 7, based on desired printing signals.
For the purpose of moving the thermal head 7 and controlling the ribbon rewind in the printer as described above, as disclosed in JP-P-62-9634-B, the thermal head is so controlled that it is moved while being thrust on the platen 2 by a predetermined distance after the point of time where the printing signal has been terminated, so that the ink ribbon is peeled off from the sheet and rewound completely and after the movement of the carriage 6 has been completely stopped, it is detached from the platen. Then in the state where the thermal head 7 is raised completely up from the platen 2, i.e. after the upward movement of the head is completely terminated, the motor 11 for driving the carriage is rotated in the reverse direction to return the carriage 6 to the home position thereof and at the same time the sheet forwarding roller 9 is rotated to forward the sheet by a predetermined distance in order to prepare the printing of the succeeding line. However, the prior art method for driving and controlling a thermal printer has a drawback in that it is unnecessarily inefficient with respect to printing speed. This inefficiency is a result of the practice of completing the upward or downward movement of the thermal head before initiating the sheet forwarding operation and/or the carriage advance/return operation. By delaying these operations until the thermal head is either completely raised or lowered, a short time delay is added to the time period required to print one line of text. The time delay increases the time necessary to print one line of text, and becomes significant when the time delays for each line are added over the period needed to print an entire page. This added time delay causes the ratio of actual printing time (i.e., the time during which the thermal head is in contact with and printing onto the printing sheet) to the total printing time (time spent printing a document) to decrease. This ratio is further increased when the line printed does not extend from margin to margin (i.e., a half line at the end of a paragraph).